Add us to not so good experience with Montessori.
My main concern is DS3. The poor kid had to keep counting from 1-10 forever because he wasn't able to write the numbers and they wouldn't move him to addition without him being able to write. All that while telling me "But none of the 3 year olds can write." Doh. No surprise the kid was bored to death and went on a few preschool strikes. They did teach him to write really well though and he can write nice numbers and letters now. Nevertheless their inability of distinguishing between fine motor skills and math skills is enough for me to feel extremely guilty about putting him there. Reading wasn't that different either, they "taught" him what he already knew.
He is going back to a regular play based preschool. A preschool where the teacher got DS5 when he was there. The teacher was so thrilled that DS3 was reading and said she would let him read to the class. Wow, nothing like that ever happened in Montessori. I don't think they even know on which level he reads (the same is true for DS5).
DS5 had a better experience since he was so clearly out of their 3-6 yr program that they did change lots of things for him. There is no excuse for having him do spelling books with "cat" and "hat" though. They did bump him up to 2nd grade books, but it's still not enough. They did better in math and taught him some new things. Right now the teacher has no idea where to take him and as far as I can tell she is not following any curriculum and jumping from one topic to another without him being ready for it and sometimes I feel the teacher is not ready herself. Don't ask. There are a few things we will have to fix once DS5 is hs next year.
Socially it was a mismatch, especially for DS5.
To sum it up. Montessori for DS5 was much better than public school would have been. Yeah, they were not clearly ready for a kid like him (they said so much), but they did try here and there. REgular K curriculum and full day would have killed him.
DS3 would have been much better of in a regular play based preschool. I like to say that no academics is better than wrong academics and this is so the case. He would have been better off playing with kids, having fun than repeating nonsense tasks over and over. He got all the academics he needed at home anyway.
